Although much has been learned about venereal syphilis in the more than one hundred years since the discovery of its etiologic agent, Treponema pallidum subsp. pallidum (Tp), the disease remains one of the most enigmatic infectious disorders of humans as well as a global threat to public health. Our limited understanding of this chronic, systemic, sexually-transmitted infection reflects the many peculiarities of the syphilis spirochete, which include an inability to replicate continuously in artificial medium, a labile outer membrane (OM), and an extremely narrow mammalian host range. Using evidence derived from multiple lines of investigation, we showed during prior funding intervals that the molecular architecture of Tp differs radically from that of protypical double-membrane organisms, such as E. coli. Our overarching objective during the current interval has been to extend this ultrastructural model into a more textured and cohesive understanding of how the syphilis spirochete sustains itself within the hostile milieu of the human host. We have pursued two parallel lines of investigation to accomplish this goal. First, we have made strong advances in our longstanding quest for Tp OM proteins. Second, we have been addressing the physiological conundrums raised jointly by our ultrastructural model and the Tp genomic sequence. Our analysis of the concealed OM lipoprotein TP0453 has helped us to unify this two-pronged approach. In addition to being the only protein unambiguously localized to the Tp OM, we now hypothesize that TP0453 represents a novel type of OM transporter. Moreover, using improved algorithms for cellular localization and structural analysis of bacterial proteins, we have compiled a roster of candidate OM-spanning proteins for molecular characterization and cellular localization; we hypothesize that one or more of these molecules form the particles visualized in freeze-fractured Tp OMs and are likely targets of opsonic antibodies. Lastly, we have made considerable progress in branching out from our traditional areas of emphasis to an under-studied but highly disease-relevant facet of treponemal physiology-the competition between pathogen and host for sequestered transition metals. From these latter studies we now postulate that the syphilis spirochete maintains transition metal homeostasis via the coordinated expression of its general metal (Tro) and zinc-specific (Znu) ABC transporters. In this application, we propose to continue the process of integrating ultrastructural, physiological, and genetic information about Tp into a pathogenesis-oriented conceptual framework by (a) elucidating the mechanism by which TP0453 creates channels in the Tp OM; (b) characterizing and localizing candidate OM proteins in Tp; and (c) to garner support for our model of transition metal acquisition in Tp via the Tro and Znu ABC transporters.